As electronic equipment has recently been being made smaller and lighter in weight, and as mobile gear has been finding wider use, switching power supplies have been being made smaller with higher performance. The switching power supplies are used in various circuits needing power supply. In personal computers (PCs), for instance, DC-DC converters are mounted near digital signal processors (DSP), micro-processing units (MPU), etc. As the operation voltage of large-scale integrated circuits (LSI) constituting DSP and MPU becomes lower, measures are taken to lower the output voltage of DC-DC converters and increasing electric current thereof. Because lowering the operation voltage makes the operation of LSI unstable to the variations (ripple) of the output voltage, measures are focused on increasing the switching frequencies of the DC-DC converters.
Switching power supply circuits comprise inductance elements such as transformers, choke coils, etc. Increase in the switching frequencies results in decrease in the number of winding of a coil on a ferrite core constituting an inductance element, preferable from the aspect of the miniaturization of switching power supply circuits and the reduction of copper loss. For such purpose, too, further increase in the switching frequencies is expected.
Because the switching power supply circuits are used in various environments such as electric cars (EVs), hybrid electric cars (HEVs), mobile communications equipment such as cell phones, etc., they are subject to various ambient temperatures and loads. The switching power supply circuits may be put at temperatures near 100° C. not only by their own heat, but also by heat generated by surrounding circuits or ambient temperatures. Because such switching power supply circuits are used at high frequencies in various environments, ferrite cores therein are demanded to have low power loss at a high frequency in a wide temperature range and a wide operating magnetic flux density range. Namely, it is required that they are not magnetically saturated without subject to high electric current.
The power loss of ferrite includes eddy current loss, hysteresis loss and residual loss. The eddy current loss is caused by electromotive power of eddy current generated by electromagnetic induction, increasing in proportion to the square of frequency. The hysteresis loss is caused by DC hysteresis, increasing in proportion to the frequency. The residual loss is the rest of the loss caused by domain wall resonance, spontaneous resonance, diffusion resonance, etc. It is well known that the power loss varies in a secondary-curve manner relative to the temperature, usually minimum at a point that the crystal magnetic anisotropy constant K1 is 0. The temperature at which K1 is 0 is a temperature at which the initial permeability μi is the maximum. Thus, it is called the secondary peak of the initial permeability μi.
The Mn—Zn ferrite cores having high saturation magnetic flux densities are used for switching power supply circuits such that they have low power loss in various environments. However, the Mn—Zn ferrite containing more than 50% by mol of Fe2O3 has extremely smaller volume resistivity because of the existence of Fe2+ in the spinel than that of Ni—Zn ferrite, so that it has larger power loss due to eddy current loss as the switching frequency becomes higher. Accordingly, the switching power supply circuit comprising the Mn—Zn ferrite exhibits efficiency decreasing as the frequency increases.
To reduce the power loss of ferrite, various methods have been proposed so far. To reduce the power loss at high frequencies, it is effective, for instance, to reduce the crystal grain size of ferrite, and to form an insulating grain boundary phase containing high-resistance Si and Ca. As such methods, Matsuo et al., “Loss Reduction of Mn—Zn ferrite,” The Journal of The Magnetics Society of Japan, Vol. 20, No. 2, 1996, pp. 429-432 propose to increase the concentration of Ca in grain boundaries by the addition of alkali metal chlorides to provide them with high AC resistivity, thereby reducing the power loss at high frequencies.
Minagawa et al., “Power Loss of Mn—Zn Ferrite Containing SnO2,” The Journal of The Magnetics Society of Japan, Vol. 20, No. 2, 1996, pp. 497-500, propose to suppress the movement of electrons between Fe2+ and Fe3+ by substituting part of Fe with Sn to increase resistance in the crystal grains, thereby reducing the eddy current loss.
Matsuya et al., “Ultra-Low-Loss Ferrite Materials,” Power and Powder Metallurgy, Vol. 41, No. 1, propose that because the power loss at a frequency exceeding 500 kHz is predominantly residual loss, the residual loss is reduced by preventing domain wall resonance, which is achieved by making the crystal grain structure finer (as small as 3-5 μm) to reduce domain walls.
JP 08-001844B proposes the addition of Co having a positive crystal magnetic anisotropy constant to reduce the temperature dependency of power loss, and Si, Ca and Ta to reduce the eddy current loss, thereby providing the Mn—Zn ferrite with low power loss at as high a frequency as 500 kHz or more in a wide temperature range of 20° C.-120° C.
The power loss of Mn—Zn ferrite has been reduced to some extent by various proposals as described above. Because the efficiency of the switching power supply is largely affected by the power loss of a ferrite core, it is necessary to further reduce the power loss of the ferrite core to provide the switching power supply with higher efficiency. Now that the switching frequency of the switching power supply has increased to 1-2 MHz, and is further proposed to as high as about 4 MHz, demand is mounting on Mn—Zn ferrite having low loss even in such a high switching frequency in a wide temperature range, as well as a high saturation magnetic flux density. However, the above conventional Mn—Zn ferrites fail to meet such demand.